Cavity ring-down spectroscopy (CRDS) systems include a cavity that reflects light within the cavity to provide a long path through a sample. The decay of light intensity from the cavity over time may be used to determine the strength of absorption of the sample. From a known absorption cross section at a probe wavelength, and the cavity loss from the sample measured by a ring-down time, a measurement of the sample, such as gas concentration, may be obtained. While achieving the input and output through a single mirror has significant benefits in terms of light intensity, some systems keep the cavity in resonance with a laser wavelength by tuning the path length using a piezoelectrically driven tuning mirror. Since a reflected portion of input light directly is coincident with ambient light, the former can perturb the measurement of cavity light and make it difficult to determine when light is coupled into the cavity or just reflected onto the detector without reaching the cavity.